FIG. 1 is an oblique view of a case for a personal digital assistant showing our new design; FIG. 2 is a left side elevational view thereof, the right side being a mirror image of the left side; FIG. 3 is a top plan view thereof; FIG. 4 is a front elevational view thereof; FIG. 5 is a rear elevational view thereof; and, FIG. 6 is a bottom plan view thereof. The broken lines are shown in the views for illustrative purposes only and form no part of the claimed design.